Epithelial skin cancer is currently the most prevalent form of cancer in humans, with over 700,000 new cases reported each year in the US alone. Sun-exposure is clearly a risk factor for all types of skin cancers, especially non-melanoma skin cancers. Current strategies to combat this growing epidemic have centered on public education and early detection. The objectives of this study are: (a) to develop a biomarker for measuring cumulative exposure to ultraviolet radiation (UVR) and (b) to determine the validity of this biomarker in predicting the risk of developing non- melanoma skin cancer. The biomarker for UVR exposure will be based on quantifying UVR-specific mutations in DNA isolated from skin biopsies. The development of a true molecular dosimeter for cumulative exposure to UVR (i.e., cumulative over the long lifetime of basal skin cells) requires that UVR-specific mutations be quantified in a conserved DNA sequence that can tolerate mutations with no loss of cellular function. The proposed target for mutation detection is the 2000 copy 5S rRNA gene, in which numerous stable mutations have been described with no detectable effects on cells. The mutations specific to sun-exposure will be determined from a comparison of mutation spectra in the 5S rRNA gene from DNA of both sun- exposed and unexposed skin from non-melanoma skin cancer patients. Mutations will be measured by gap-LCR, PCR or other high-sensitivity techniques that allow for accurate quantification. To validate the 5S rRNA mutation biomarker for non-melanoma skin cancer, skin biopsies from sun-exposed and non-exposed areas will be obtained from non-melanoma skin cancer patients (squamous and basal cell carcinomas) and age/race/sex matched controls within skin tumors. Mutation levels in the 5S rRNA gene will be correlated with both the occurrence of cancer and specific mutations in the p53 gene (formed in greater than or equal to 90% of squamous cell and 56% of basal cell carcinomas). The critical difference between this proposed study and studies that measure UVR-specific mutations in critical oncogenes or tumor suppressor genes linked to skin cancers (ras, p53, MTS1, etc.) is that mutations in these genes indicate that these cells are in the process of being transformed, and therefore, are of limited use in determining risk prior to tumor formation. The development of a quantitative dosimeter for sun-exposure would provide a screen to determine relative risk of non-melanoma skin cancer well before critical genes are mutated. Such a dosimeter may also be useful in determining the role and timing of p53 mutations in non-melanoma skin cancer development and the effectiveness of sunscreens in preventing DNA mutations and skin cancer.